Active beam delivery system with variable optical path segment through air

ABSTRACT

A laser energy delivery system includes a relay imaging system. Input optics arranged to receive the laser energy, a transmitting mirror having adjustable angle of incidence relative to the input optics, and a robot mounted optical assembly are configured to direct laser energy toward the movable target image plane. The laser energy follows an optical path including an essentially straight segment from the transmitting mirror to the receiving mirror, having a variable length and a variable angle relative to the input optics through air. Diagnostics on the processing head facilitate operation.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/990,861 filed on 17 Nov. 2004 (now U.S. Pat No. 7,718,921), whichapplication is incorporated herein by reference.

The present application is related to co-pending U.S. patentapplications, which have common inventors and common assignees with thepresent application, including the following:

U.S. Patent Application Publication No. US-2006-0102609-A1 (now U.S. PatNo. 7,750,266); U.S. patent application Ser. No. 10/990,991; entitled“Active Beam Delivery System for Laser Peening and Laser PeeningMethod,” by Dane et al., filed 17 Nov. 2004.

U.S. Patent Application Publication No. US-2006-0102604-A1 (now U.S. PatNo. 7,851,725); U.S. patent application Ser. No. 10/990,992; entitled“Active Beam Delivery System with Image Relay” by Dane et al.; filed 17Nov. 2004.

The Examiner's attention is drawn to the prosecution of the relatedcases.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high energy laser systems, to beamdelivery systems, and to laser peening systems suitable for use withstationary targets.

2. Description of Related Art

The use of mechanical shocks to form metals and to improve their surfaceproperties has been realized for ages. In current industrial practice, apeening treatment of metal surfaces is accomplished by using highvelocity shot. Treatment improves surface properties and, veryimportantly for many applications, results in a part displayingsignificantly improved resistance to fatigue and corrosion failure. Awide range of work pieces are shot peened in the aerospace andautomotive industries. However, for many applications, shot peening doesnot provide sufficiently intense or deep treatment, or cannot be usedbecause of its detrimental effect on the surface finish.

With the invention of the laser, it was rapidly recognized that theintense shocks required for peening could be achieved by means of alaser-driven, tamped plasma. B. P. Fairand, et al., “Laser Shot InducedMicrostructural and Mechanical Property Changes in 7075 Aluminum,”Journal of Applied Physics, Vol. 43, No. 9, p. 3893, September 1972.Typically, a plasma shock of 10 kB to 30 kB is generated at metalsurfaces by means of high energy density (about 200 j/cm²), short pulselength (about 30 nanoseconds) lasers. A thin layer of metal tape, blackpaint or other absorbing material on the metal surface provides anabsorber to prevent ablation of the metal. A confining or tampingmaterial such as water covers the surface layer providing an increasedintensity shock. These shocks have been shown to impart compressivestresses, deeper and more intense, than standard shot peening. Intesting, this treatment has been shown to be superior for strengtheningwork pieces from fatigue and corrosion failure. Laser peening is alsoused for forming and texturing surfaces.

One laser system which has been utilized for this purpose is describedin our prior U.S. Pat. No. 5,239,408, entitled HIGH POWER, HIGH BEAMQUALITY REGENERATIVE AMPLIFIER. The laser system described in the justcited '408 patent comprises a high power amplifier in a masteroscillator/power amplifier MOPA configuration capable of producingoutput pulses greater than 20 Joules per pulse with the pulse width onthe order of 10 to 30 nanoseconds or less using a wavefront correctingconfiguration based on a stimulated Brillouin scattering SBS phaseconjugator/mirror system.

In current commercial high energy laser peening processes, the laserbeam position is held at a fixed location. The work piece being treatedis moved through the laser beam to create the applied spot pattern whilemaintaining the desired incidence angles, spot sizes, and orientations.This requires automation and work piece holding fixtures to grip thework piece and move it through its programmed positions. This methodbecomes both costly and highly work piece specific, requiringconsiderable engineering to develop processes for new work pieces. Inaddition, work piece size is limited to the lifting and handlingcapacity of the automation utilized. Work pieces and structures largerthan automation handling capacity (for example, >1 m and/or >100 kg)cannot be laser peened by the conventional work piece moving approach.

Flexible beam delivery systems are often based on the use of opticalfibers. However, even at wavelengths where glass fiber transmission isnormally high, the very high pulse energy and high peak power used inlaser peening can damage the fibers and render them ineffective. Forexample, a 25J pulse is 100 times the maximum pulse energy (250 mJ) thatcan be delivered through a 1 mm multi-mode fiber. For single frequencybeams, such as used in representative laser peening applications, glassfibers have even lower damage thresholds.

Approaches to actively scanning a laser beam for the delivery of highpower carbon dioxide (CO₂) cutting and welding lasers have beendeveloped. Because of its 10 μm wavelength in the far infrared, theoutput of a CO₂ laser cannot be delivered by glass fibers. Commercialarticulated arms have been developed for high power CO₂ lasers thatconsist of a series of hollow tubes connected by seven articulationpoints (commonly called knuckles), each of which houses a 45 degreemirror. There are a number of important drawbacks to an articulated armfor laser peening that lead us to develop an alternative approach:

1. Beam rotation—For laser peening, it is desirable to use a square beam(unlike CO₂ lasers) and the out-of-plane reflection at each articulatedjoint would cause some degree of beam rotation. Although this could becompensated by appropriately rotating the square beam before it entersthe arm, the precise orientation of each arm segment would need to beknown. Since there are multiple arm positions for a given delivery angleto the part, each of the seven rotational joints would need to beaccurately encoded.

2. Pointing accuracy—The arms in the apertures needed to transmit apeening beam typically have a pointing accuracy of only 1 mrad,corresponding to up to 1 mm error in the positioning of a 3 mm spot, asused for example in laser peening.

3. Optical losses—A standard seven-knuckle articulated arm would requireseven mirror reflections between the input and the output, introducingoptical losses during beam delivery that reduce efficiency of thesystem.

4. Length limitations—Articulated arms have a fixed length, allowinglimited flexibility as to placement with respect to the work piece. Themaximum delivery length would also be limited by the weight andmechanical stiffness of tubular arm segments and the bearing loads ateach joint.

5. Process development—Typically, the articulated arm is designed to bequite flexible; its design under-constrained so that multiple knuckleconfigurations are possible for a given treatment spot. However, it isstill possible to damage the arm by forcing it through disallowed pathsor by causing collisions with the process robot. For this reason, muchof the complex robot path development already associated with the movingpart process would still be needed.

It is desirable to provide a system that provides sufficient flexibilityto be able to treat large work pieces and work pieces “in situ” atcustomer facilities, like aircraft parts at an aviation repair stationor large oil drilling work pieces at a pipe yard.

SUMMARY OF THE INVENTION

A laser peening method and system, which allow the work piece to befixed, while moving and directing the laser beam in a highly controlledmanner are described. Utilizing a fixed work piece position minimizesholding fixture and work piece moving complexity, thereby reducing costand engineering. Moving the laser beam rather than the work piece, orwith limited movement of the work piece, will allow work pieces largerthan automation handling capacity to be cost effectively laser peened.

A method and system for delivering laser energy, including laser energyhaving high power such as used in laser peening, is provided. Anembodiment of a laser energy delivery system for high power laser energyincludes a relay imaging system which relays an image of an output ofthe source of laser energy to the target image plane near the surface ofthe work piece. The relay imaging system includes input optics arrangedto receive the laser energy from the output of the source of laserenergy, a transmitting mirror having adjustable angle of incidencerelative to the input optics, and a robot mounted optical assemblydirecting laser energy toward the target image plane. The robot mountedoptical assembly includes a receiving mirror having adjustable angle ofincidence relative to the transmitting mirror and output optics whichcondition the laser energy for the target surface. The laser energyfollows an optical path including an essentially straight segment fromthe transmitting mirror to the receiving mirror. This segment has avariable length and variable angle relative to the input optics. As arobot mounted optical assembly is positioned to point the laser beamonto target locations on the target surface, the transmitting mirror andreceiving mirror angles are adjusted, varying the length and angle ofthe segment of the optical path between them, to couple the laser energyfrom the input optics to the output optics through air. The input andoutput optics perform the image relay, positioning a near field image ofthe output of the source of laser energy at the target image plane. Therobot mounted optical assembly is manipulated to position the laser beamso that the target image plane lies within a range of the target surfaceof the work piece, so that the beam shape at the target surface is closeto the beam shape near the output of the source of laser energy.

A robotic system for delivering laser energy from a source of laserenergy to a target surface on the work piece is provided suitable foruse in laser peening, and for other uses. An embodiment of such systemincludes beam delivery optics having adjustable components arranged toreceive the laser energy from an output of the source of laser energyand to direct the laser energy along an optical path toward the targetsurface. The optical path includes a variable segment between componentsof the beam delivery optics having a variable length and variabledirection through air. The beam delivery optics establish an output beamline for the laser energy. The controller is coupled to the adjustablecomponents of the beam delivery optics to move the output beam line forthe laser energy, continuously or in a stepwise fashion, among targetpositions on the target surface. Diagnostic sensors are provided withthe beam delivery optics in embodiments of the technology, and theinformation provided by such sensors can be fed back to the controllerfor precise robotic control of the system and other diagnosticfunctions.

In embodiments of the invention, the beam delivery optics include inputoptics arranged to receive the laser energy from an output of the sourceof laser energy, and to direct laser energy on a first optical pathsegment. A transmitting mirror having an adjustable angle of incidencerelative to the first optical path segment reflects the laser energy onthe second optical path segment, which comprises the variable segmentmentioned above, of the optical path. A robot mounted optical assemblyincludes a receiving mirror adapted to be positioned in the secondoptical path segment. The receiving mirror has an adjustable angle ofincidence relative to the second optical path segment to reflect thelaser energy on a third optical path segment. An output telescope isincluded in the robot mounted optical assembly and positioned in thethird optical path segment. The output telescope directs the laserenergy toward the target surface on the output beam line. The inputoptics in embodiments of the system include a telescope arranged toproject the laser energy along the first optical path, and to enlargethe cross-section of the laser energy for propagation across thevariable length segment through air.

Also, in embodiments of the system, optics are provided for rotatingcross-sections of the laser energy to offset rotation caused by the beamdelivery optics and maintain consistent orientation of the laser energyon the target surface. Thus, in an embodiment of the system, thecross-section of the laser energy is rectangular, and reflections off ofthe transmitting and receiving mirrors at variable angles tend to causerotation of the rectangle. In one embodiment, first and second pulses oflaser energy are directed on first and second beam paths for delivery tothe target surface, and the first and second beam paths have respectiveincident and reflected beam lines at the transmitting mirror andrespective incident and reflected beam lines at the receiving mirror.Optics are provided for rotating the cross-section of the first andsecond pulses according to an angle between the plane containing theincident and reflected beam lines on the transmitting mirror and a planecontaining incident and reflected beam lines on the receiving mirror.

Other aspects and advantages of the present invention can be seen onreview of the drawings, the detailed description and the claims, whichfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an energy delivery system configured for laserpeening a work piece.

FIG. 2 is a diagram of an energy delivery system configured in laserpeening a work piece in situ.

FIG. 3 is a diagram of input optics and a transmitting mirror for energydelivery systems like those of FIGS. 1 and 2.

FIG. 4 is a diagram of robot mounted optical assembly, including areceiving mirror and output optics for energy delivery systems likethose of FIGS. 1 and 2.

FIGS. 5 and 6 are simplified diagrams of energy delivery systems likethose of FIGS. 1 and 2.

FIG. 7 is a diagram of a source of laser energy configured for use incombination with energy delivery systems like those of FIGS. 1 and 2.

DETAILED DESCRIPTION

A detailed description of embodiments of the present invention isprovided with reference to the FIGS. 1-7. The articulated segments andseven mirrors of a conventional energy delivery system are replaced inembodiments of the energy delivery system described herein, with twomirrors on high-speed, high-resolution gimbals. Propagation to a laserpeening processing head that comprises a robot mounted optical assemblyis by a free air path between the two gimbal-mounted mirrors. Beamdiagnostics also located on the processing head monitor laser pointing,beam rotation, and laser energy on a shot-to-shot basis. Finally, aprocessing camera, located on the output of the tool, images the laserpeening treatment plane.

FIG. 1 shows a schematic of a laser peening system. This is not meant tobe a scale design of an actual system but illustrates basic componentsand their layout. The system of FIG. 1 includes a laser 100 in a masteroscillator/power amplifier configuration, such as illustrated in FIG. 7,or other laser energy source. The laser 100 is mounted on a stable lasertable 101. Output from the source of laser energy is applied to inputoptics 102 which condition the beam for delivery through a relaytelescope 103 to a transmitting mirror 105A mounted on transmittingmirror gimbal 105. The transmitting mirror 105A reflects the beam to areceiving mirror 106A mounted on receiving mirror gimbal 106. Thereceiving mirror 106A on receiving mirror gimbal 106 is part of a robotmounted optical assembly 107, which is in turn positioned by robot 108.The robot mounted optical assembly 107 includes output optics fordirecting the beam to a target location on a surface of a work piece109. In this embodiment, the work piece 109 is mounted on a rotatableparts holder 110. A water delivery robot 111 is mounted near the partsholder 110, and includes a vessel 111A for delivery of tamping fluid inthe laser peening application. The robot 111 in embodiments of thetechnology also controls placement of a coordinate measuring metrologytouch probe (such as the Renishaw style) used during laser peeningoperations. A controller 112 for the robot 111, a controller 113 for therobot 108, and a controller 114 for coordinating operation of the robotsand adjustable components in the beam delivery system and in the laser100, and other controllable components are provided with the system.

The basic optical path from the input optics 102 to the target workpiece includes just two turns in this embodiment, which are controlledusing high-speed, high-resolution gimbals. The optical path includes asegment 120, between the transmitting mirror 105A and the receivingmirror 106A, which is essentially straight and has a variable lengththrough air, and a variable direction defined by the angle setting ofthe transmitting mirror gimbal. The variable length is controlled by therobot 108 based on the positioning of the optical assembly 107 whenmoving the beam line to a target location on the surface of the workpiece 109. Likewise, the variable direction is set using the gimbals105, 106 according to the positioning of the optical assembly 107. Inthe embodiment illustrated, the segment 120 extends through free air,that is without an enclosure such as a tube. In other embodiments, atelescoping tube or other enclosure could be provided so long as it issufficiently adjustable.

The water robot 111 is used to deliver the transparent tamping layer tothe surface of the treated part. An alternative system integrates awater delivery vessel on to the robot 108 along with the robot mountedoptical assembly 107.

A process chamber 130 is illustrated, including an access door 131 fortechnicians, a parts access door 132 which allows access to the partsholder 110, and a shutter 104 for admitting the laser radiation. Theprocess chamber 130 allows provision of a controlled environment for theoperation of the robot 108, with a parts holder 110 used to provide onlylimited positioning functions for the laser peening operation. Theprocess chamber 130 is mounted on a platform, such as a foundation ormovable plank, and the transmitting mirror gimbal 105, robot 108 withthe robot mounted optical assembly 107, the robot 111 and the rotatableparts holder 110 are all mounted thereon in a fixed spatialrelationship. The laser 100 and input optics 102 are mounted on separatestages, closely coupled with the process chamber 130.

FIG. 2 illustrates a configuration for the treatment of a very largepart, such as in laser peen forming work pieces in situ mounted on largestructures. In the illustrated embodiment, the robot mounted opticalassembly 201 and the water robot 202 are mounted onto a common processplatform 200 that can be moved along a set of precision tracks 219 toaccess different portions of an extended part 218, such as a wing skinon an airplane. The free air propagation of the treatment beam along thesegment 220 from the transmitting mirror gimbal 221 to the receivingmirror gimbal 222, allow a flexible distance between the laser sourcesupplying the input 223 at the transmitting mirror and the robot mountedoptical assembly 201. The driving mechanism, such as a screw drive 230,is coupled to the platform 200, and allows for precision movement of theplatform 200 adjacent a large object. Utilities including air, water andelectrical conduits and the like are provided in a utilities trackway231 along the tracks 219. The controller comprising the water robotcontroller 235, the controller 236 for the robot used for positioning ofthe optical assembly 201, and other control logic 237 are coupled to theutilities and to the devices on the track. Input optics are coupled to ashutter 238 between the output of the laser and the transmitting mirrorgimbal 221.

In this case, the robot mounted optical assembly and the water robot aremounted onto a common platform that can be moved along a set ofprecision tracks to access different portions of an extended part, suchas a wing skin. The active beam delivery system uses free airpropagation of the treatment beam, allowing a flexible distance betweenthe laser source and the treatment tool. The incorporation of bothrobots onto a single platform enables the delivery of laser energy atlocations difficult to reach otherwise. Examples of this could be anelevated platform at the level of an airplane wing or vertical finattachment bulkhead. The laser source can remain securely installed atground level, feeding laser light to the remote treatment location. Forextended propagation distances between the laser and the robot mountedoptical assembly, e.g. greater than 5 m, the relay optics can bemodified on the laser system.

FIG. 3 illustrates a layout for input optics up to the transmittingmirror, labeled M55 in FIG. 3. Laser source 300 provides an output beamon line 301 defining a first segment of the optical path. Mirror M50reflects the beam on line 302 defining a second segment of the opticalpath to active alignment optics which comprise alignment laser AL50,half wave plate WP50, lens L50, polarizer P50, lens L51 and camera C50.The beam which propagates through the polarizer P50 proceeds on a thirdsegment of the optical path along line 303 through wave plate WP51 tofield rotator optics which comprise mirror M51, mirror M52 and mirrorM53. From mirror M53, the beam as rotated propagates on a fourth segmentof the optical path on line 304 to mirror M54. Mirror M54 turns the beamthrough a beam transport telescope (also called relay telescope) whichcomprises lens L52 and lens L53, on a fifth segment of the optical pathalong line 305 to the gimbal-mounted transmitting mirror M55. WindowsW50 and W51 define the input and output of a vacuum chamber (not shown)for the telescope, in which the beam is brought through a focus. Thetransmitting mirror M55 turns the beam on a variable angle along a sixthsegment of the optical path on line 306, which is directed at thereceiving mirror on the robot mounted optical assembly as describedabove, through a variable length of air.

The alignment laser AL50 in one embodiment comprises a continuous-wave(CW, i.e. non-pulsed) laser to verify correct alignment and, ifnecessary, to enable feedback adjustments to the alignment in betweenlaser shots. In one embodiment, the alignment laser AL50 comprises adiode-pumped Nd:YLF laser which produces relatively low output power(<500 mW). The alignment laser AL50 has the same wavelength as thepeening laser 300, or is otherwise configured so that the reflecting andfocusing properties of the alignment beam through all of the optics canbe reliably used for alignment of the high power beam.

The divergent output from alignment laser AL50 (<500 mW) is collimatedby lens L50 and combined with the high power beam path at polarizingbeam splitter P50. Using half waveplate WP50, the polarization of thealignment laser is set to S-polarization so that it reflects at thepolarizer on the beam line 303. A small portion of the high power beamtransmitted in P-polarization is reflected at the polarizer P50, and asmall portion of the alignment beam is transmitted through polarizer P50to the camera C50. Diagnostic camera C50 detects the positions of thealignment and high power beams, and provides feedback for achievingprecise co-alignment. The camera is placed at the focus of lens L51.When the far field (focal point) of the small leakage of the high powerbeam reflected from the surface of polarizer P50 precisely overlaps thefocal point of a portion of the alignment beam that transmits throughthe polarizer P50, then co-alignment is confirmed. Waveplate WP50 can berotated to allow the fraction of alignment beam transmission through thepolarizer P50 to be adjusted.

In embodiments of the system in which the output of the high power laseris not round, rotation of the cross-section of the beam caused by thetransmitting and receiving mirrors is compensated in the field rotatoroptics. For example, in a laser peening system, a square beamcross-section, or other rectangular shape, is preferred. Depending onthe relative angle between the plane containing the incident andreflected beams on the gimbal-mounted transmitting mirror M55 and theplane containing the incident and reflected beams on the gimbal-mountedreceiving mirror M56 (see FIG. 4), the square beam will be rotated withrespect to the coordinates of the robot mounted optical assembly. Thefield rotator optics pre-rotate the beam cross-section so that thedesired spot orientation is delivered to the target surface. The fieldrotator optics consist of three mirrors M51-53 which are rigidly mountedon a common structure which can rotate around the input beam axis usinga remotely controlled rotational stage. Since there is an odd number ofreflections (3), rotating this three mirror assembly will cause thesquare beam to rotate at 2× the rate, i.e. a 45 degree mirror assemblyrotation will cause a full 90 degree beam rotation. In the case of asquare beam, a±22.5 degree rotation of the field rotator will provideall required beam orientations. Other optical arrangements can beutilized for providing field rotation.

It may be desirable, e.g. for off-axis peening, that the polarizationstate of the beam not be orthogonally aligned to the square beam shape.Waveplate WP51, placed in the high power beam path, will allow thepolarization to be rotated to an arbitrary linear state. Like the fieldrotator, it will be mounted in a remotely-controlled rotational stageand the polarization will rotate at 2× the rate of rotation of thestage.

The transport telescope, formed from lenses L52 and L53, serves toenlarge the square beam and to relay an optical image across thefree-propagation path to the processing head comprising the robotmounted optical assembly. Through this telescope, the beam is magnifiedin one embodiment by about 1.4× from a nominal dimension of 23 mm squareto 32.5 mm. This has three functions. The first is that the beam area isincreased by 2× on the transmitter and receiver mirrors, lessening therisk of optical damage. The second function is that the relay distanceof the telescope is increased by the magnification squared (i.e. 2×)making it possible to provide a well defined beam image at the distanttreatment plane. Finally, magnifying the beam increases the Raleighrange (defined as twice the confocal parameter) by 2× with a 1.4 timesmagnification, improving the free-space propagation characteristics ofthe beam. This third function is important since the optical relaytelescope and the beam delivery telescope in the processing head havebeen optimized for a single propagation distance. However, as theprocessing head is maneuvered within a±45 degree processing solid angle,the actual propagation distance between the gimbals can vary by up to ±1m. This variation can be even larger in the case of the arrangement forin situ laser peening of large parts as shown in FIG. 2.

The transmitter and receiver gimbals are of similar design andspecifications in an embodiment of the system. The motor for arepresentative system in each axis has a resolution of 25 μrad (5.2arcsec), a repeatability of 50 μrad (10.3 arcsec), and an absoluteaccuracy of 100 μrad (20.6 arcsec). These specifications are for theactual reflected beam; the values for the mirror angles are 2× smaller.The transmitter and receiver mirrors are 4″ in diameter in arepresentative embodiment, and have a high damage threshold coating thatefficiently reflects the beam over an angle of incidence range of 15-55degrees.

FIG. 4 illustrates the receiving mirror M56, and other optics in therobot mounted optical assembly on the processing head in an embodimentof the system including input optics of FIG. 3. The receiving mirror M56accepts the beam on beam line 306 from the transmitting mirror M55 (FIG.3). The receiving mirror M56 is adjusted to turn the beam on a seventhsegment of the optical path on line 407 toward diagnostic beam splitterDS50. The beam propagates through the diagnostic beam splitter DS50 toan output telescope comprising lenses L57 and L58. The combination ofthe beam transport telescope and input optics, and the output telescopeon the processing head establish a target image plane 410 for a 3 mmspot at a location 79 cm from the final lens L58 in one embodiment.

The relay imaging system illustrated in FIGS. 3 and 4 places a precise,demagnified image of the near field output beam from the laser near thesurface of the part. Using a laser source like that shown in FIG. 7,which produces a highly uniform, flat-topped irradiance profile in thenear field output of the laser, a highly uniform, flat-topped irradianceprofile, free from significant optical diffraction structure otherwisecaused by propagation away from the source, is projected into a targetimage plane that lies within a determinate range of the target surfaceof the work piece. The irradiance profile on the target surface of thework piece within a determinate range of this target image planemaintains substantially the same quality as the near field image fromthe output of the laser. The range allowed around the target image planefor placement of the work piece depends on the parameters of theoperation being performed, and can be for example within plus or minusone meter of the target image plane in a particular embodiment. In otherembodiments, lesser or greater determinate ranges for placement of thetarget surface on the work piece relative to the target image plane aresuitable, depending on the characteristics of the laser energy, therequirements of the function being performed by the laser energy andother factors.

Beam diagnostics on the processing head provide for sensing shot-to-shotenergy measurements, alignment diagnostics, and output beam profilediagnostics. Diagnostic beam splitter DS50 directs a small fraction(about 0.8% for example) of the incoming beam to the diagnosticcomponents on line 403 through lens L54, and diagnostic beam splitterDS51. A calibrated pyroelectric energy meter ED50 placed in the beamwhich propagates through the diagnostic beam splitter DS51 will provideshot-to-shot energy measurements at the processing head. The diagnosticbeam splitter DS51 directs a portion of this beam to alignmentdiagnostics including diagnostic beam splitter DS52, optical shutterOS50, diagnostic beam splitter DS53, lens L55, camera C51, camera C52,lens L56, mirror M57 and camera C53. The telescope consisting of lensesL54 and L56 forms an image of the high power square beam at the outputaperture of the laser on camera C53. This also corresponds to thespatial profile, scaled in size, at the treatment plane on the surfaceof the part. Lenses L54 and L55 place an image of the beam at the planeof the receiver gimbal mirror M56 onto camera C51. Camera C52 is placedat the focus (far field) of lens L54 so that the position of thealignment beam on this camera indicates the pointing angle from thereceiver gimbal. The optical shutter OS50 is closed during high energypeening in order to protect the alignment cameras from the high powerbeam since C51 and C52 are set up to be used with the low power CWalignment beam.

For the purpose of context, in a system utilized for laser peeningobjects mounted on a fixed stage, or on a rotating stage, as describedabove, the distance from the receiver gimbal and the target plane in atypical system may be from about 0.5 to about 1.5 meters. The distancehowever can be longer or as shorter, depending on the particular use ofthe beam delivery system and practical limitations on sized ofcomponents.

The rotational parts stage shown in FIG. 1 comprises, for example, asingle axis rotary stage controlled by the processing robotics system.An embodiment of the active beam delivery system is configured todeliver the beam within a±45 degree solid angle (0.5π steradians) over atreatment volume of 50×50×50 cm³. This treatment volume can be furtherexpanded by limiting the angular range (i.e. incident angles closer tonormal) or by moving the entire robotic platform as discussedpreviously. Another method of simplifying the required robot movement isto place the work piece to be processed on a simple rotary stage that isdriven by the robot controller as a “seventh axis.” By transferringangular motions that would be required by the beam in the horizontalplane to rotation of the part, additional flexibility is provided. Inthis way, the processing head angular motion can be primarily in and outof the plane of the drawing in FIG. 4 with horizontal angles provided bypart rotation. This provides access to other surfaces of a multi-sidedpart and will also be particularly applicable to parts with circularlysymmetric cross-sections such as gears and integrated blade rotors.

FIG. 5 and FIG. 6 provide perspective views of a laser energy deliverysystem implemented as described above. The input laser beam on line 500is projected by the input optics to the mirror on transmitter gimbal501. The mirror on transmitter gimbal 501 reflects the beam across thevariable length of air at a variable angle on line 502 to the mirrormounted on the receiver gimbal 503. The beam is propagated through theprocessing head 504 including diagnostics and an output telescope onbeam line 507 to a target work piece 505. Robot 506 positions theprocessing head 504 for delivery of the beam. The receiver gimbal 503and the transmitter gimbal 501 are coordinated to deliver the laserradiation to the processing head 504. As illustrated in FIG. 6, as therobot 506 repositions the processing head 504, the transmitter gimbal501 is operated in coordination with the receiver gimbal 503 to directthe beam to the target location. As can be seen, the beam on beam lines500 and 502 lies in a plane (508) the orientation of which is defined bythe incident and reflected angles on the mirror mounted on thetransmitter gimbal 501. Also, the beam on the beam lines 502 and 507lies in a plane (509) the orientation of which is defined by theincident and reflected angles on the mirror mounted in the receivergimbal 503. As the robot positions the processing head 504, these planesrotate, and the direction of the beam line 502 and the distance betweenthe transmitter gimbal 501 and receiver gimbal 503 change.

The automation of the active beam delivery system including thetransmitter and receiver gimbals can be accomplished with a softwarecontrolled robot system including a program acting as a “controller incharge,” executing the laser peening process by manipulating the beamdelivery tool. A previously defined process map for a given part istraversed by the controller, which will fire the laser, as needed. Ahigher level system can be configured to transfer process control fromthe robot system to a central controller. This controller would directthe laser (via fire triggers), beam delivery gimbals, as well as the twoprocess robots. Other control system configurations can be applied assuits the particular embodiment, including for example a remote computerfor centralized actuation of in situ processing.

The process variables for each laser spot on a part consist of a (x, y,z) target location, an incidence angle (θ, φ) on the target location,the square beam rotation parameter, and the distance of the processinghead from the treatment surface (determining spot size). As the robotcontroller (or a higher level central controller) prepares to move theprocessing head to the next processing spot, it broadcasts theparameters to the gimbal controller logic so that appropriateadjustments to the transmitter and receiver gimbal angles, as well asthe field and polarization rotations, can be made in coordination withrobot motion. Based on the calibrated position of the robot, theparameters should include the computed (x, y, z) position of the centerof the receiver gimbal, the (θ, φ) angle to the part, and the squarebeam rotation. When the gimbal controller logic has finished the moveand the gimbals have settled, it will notify the robot controller whichcan then fire the laser and move to the next processing spot.

In order to direct the high power beam from the transmitter gimbalmirror to the center of the receiving gimbal mirror and then to orientthe receiving gimbal mirror to deliver the beam accurately down theoptical axis of the processing head, the gimbal controls are calibratedto the coordinate system used by the robot controller. Mapping of thecoordinate system can be done, for example, by causing the robotcontroller to step through a known set of calibration positions, basedon its own coordinate system. At each position, the beam can firstmanually, then under feedback control, be optimally directed to matcheach position. From the position data broadcast by the robot controllerfor each point and the gimbal angles required to match these positions,a consistent coordinate system can be constructed for the gimbalcontroller logic.

As described above, there are four calibration cameras mounted to theprocessing head. Each of these has a separate alignment role. There aretwo representative modes of operation using the alignment cameras. Inthe first, the cameras will be used only to periodically confirm correctbeam alignment to the processing head optical axis such as during acalibration procedure or before the processing of each part or group ofspots on a part. The second mode of operation will involve closed-loopoptimization of the pointing angles in between each laser shot. Theprocess applied in a given application therefore includes a singlecalibration step using the low power laser, continuous calibrationbetween each laser shot, or some intermediate regime.

In one embodiment, the outputs of each of the four diagnostic camerasare fed into a 4-channel frame-grabber. Onboard image machine-visionprocessing will offload computational demands from the control computer,allowing maximum throughput. Each camera is capable of triggeredoperation in an embodiment of the system, so that image acquisition canbegin immediately, on demand, without the need to wait for the next CCDrefresh cycle. A description of examples of the function of each of thefour cameras is provided in the following sections.

The gimbal position camera is denoted as C51 in FIG. 4. The opticsfeeding C51 are set up to provide an image of the alignment beam at theoptical plane of the receiver gimbal mirror. In this way, the calibratedposition of the beam image on the camera provides information about thecentering of the beam on the receiver mirror M56 and is not effected bythe tilt angle on receiver mirror M56 (within the field of view of thecamera). Given a calibrated coordinate system for the gimbals, theposition of the beam on receiver mirror M56 then directly correspondsonly to the delivery angle of the transmitting gimbal mirror M55 and canbe used to correct the delivery angle from the transmitter gimbal mirrorM55.

The gimbal angle camera is denoted as C52 in FIG. 4. C52 is placedprecisely at the focal point of the lens L54. In this way, the positionof the beam image on this camera relates to the beam delivery angle fromthe receiver gimbal mirror M56 and is not affected by the position ofthe beam on the receiver gimbal mirror M56. The position of the image onthe camera can be used to correct the delivery angle from the receivergimbal mirror M56.

The near-field camera is denoted as C53 in FIG. 4. The optics feedingC53 are set up to provide an image of the high power square beam at, orin the near field of, the output aperture of the laser system. Thisimage also provides the spatial profile of the beam at the treatmentsurface. By placing the camera on the beam delivery processing head, anyproblems that could arise from optical damage in the beam delivery trainor from misalignment or beam clipping can be detected. The size of thebeam, its uniformity, and its rotational orientation for each laser shotcan be monitored.

The process camera is depicted in FIG. 4 as C54. It is mounted on theoutput of the processing head and will be fitted with a standard videoimaging lens which will have a fixed focus at the treatment plane. Itcan show a detailed view of laser peening progress. Camera C54 has manymore alternative or additional applications. In one embodiment, thecamera will be able to view both ambient visible lighting as well as thelow-power infrared alignment beam, and can be used to actively align therobot coordinate system to the optical axis of the processing head. Theprocess camera C54 is used in an embodiment to apply machine-visionanalysis to locate and calibrate the position of the part to be treated,potentially eliminating the need for coordinate measurement with a touchprobe (Renishaw style). Using calibration targets of known dimensions,analysis of recorded image dimensions will also provide the calibrationof the distance between the processing head and the treatment surface.

Treatment rates can be limited due to the time required for robot moves.The automation processes moving the beam delivery processing head,rather than the part being treated, accelerate treatment. However, someof the same fundamental mechanical limitations will remain. Using theactive beam delivery system, two representative strategies increaseproduction throughput.

First, by continuously moving the output beam line (via movement of theprocessing head) and water robots along with the beam transmitting andreceiving gimbals, the laser system, appropriately synchronized, isfired to generate the desired treatment pattern without stoppingprocessing head movement between each shot. The need for mechanicalsettling of the robots and gimbals between each pulse is eliminated orreduced, increasing throughput.

In alternative approaches, multiple laser peening spots on a target areaccessed from a processing head position by scanning only the receivermirror gimbal at the input to the processing head. The number of spotsaccessible will be limited only by the field of view of the deliveryoptics. For example, a 5×5 array of 3 mm square spots could be laid downfrom a single processing head position with appropriate selection ofoptical components. The ultimate repetition frequency of the treatmentwould be limited only by the settling time of the water layer betweenpulses. This could be a very powerful technique, recognizing that thepulse repetition frequency PRF of lasers such as shown in FIG. 7, canreach even 10 Hz for 10-20 shots at a time as long as the average PRF,averaged over a period of ˜30 s remains at 5 Hz or less. Rapid laserbursts followed by repositioning of the processing head between burstscould cover large process areas and would be very useful in laser peenforming processes.

The basic architecture of a master oscillator/power amplifierconfiguration with a regenerative laser amplifier including an SBS phaseconjugator mirror system and relay telescope with a baffle is shown inFIG. 7, that is suitable for use in systems as described herein. Theembodiment of FIG. 7 is an improved version of a similar amplifierdescribed in U.S. Pat. No. 5,239,408, which is incorporated by referenceas if fully set forth herein. The amplifier system of FIG. 7 includes arotator 740, such as a Pockels cell or Faraday rotator, a firstintra-cavity relay telescope 720, a slab-shaped gain medium 750, asecond intra-cavity relay telescope 770 and an SBS phaseconjugator/mirror system 760. The slab 750 is enclosed in a pump cavity(not shown). Two polarizers 702 and 706 are also included for capturingan input pulse, and extracting an output pulse, respectively. Sevenflat, highly reflecting mirrors 711, 712, 713, 714, 715, 716, and 717,define an optical path through the slab 750, and telescope 720,polarizer 706 and telescope 770 connect the ring to SBS phase conjugator760. An additional relay telescope 780 relays images of the near fieldoutput (a location near the output at polarizer 706) of the ringamplifier to target delivery optics not shown.

In operation, a master oscillator 708 supplies an input pulse which hasS-polarization. The pulse reflects off polarizer 702, proceeds throughan isolation rotator 740, remaining unchanged in polarization, and isfurther reflected off polarizer 706 into a ring shaped optical pathdefined by mirrors 711-717, proceeding for this ring transit in acounter-clockwise direction off of the polarizer 706.

In the ring, the beam enters the 90 degree rotator 708 which rotates thebeam by 90 degrees to the P-polarization. The pulse proceeds throughmirrors 711 and 712 along optical path 719 through relay telescope 720.

The telescope 720 includes a vacuum chamber 722 having a first lens 724mounted by a vacuum tight seal 726, and a second lens 728 mounted byvacuum tight seal 730. A baffle 729 at the telescope focal point insidethe vacuum chamber 722 blocks off angle beams and ghost reflections.

From telescope 720, the beam proceeds through mirror 713 into andthrough the slab 750 where it is reflected by mirrors 714 and 715 backthrough the slab 750. Near unity fill of the pumped volume isaccomplished by a first zig-zag pass and a second zig-zag pass which areessentially mirror images about the direction of propagation. In thisway, the second zig-zag pass will tend to extract gain from regions thatmay have been missed in the first pass.

From slab 750, the beam is reflected off mirror 716 along path 742through telescope 720, off mirror 717 where it is reflected back intopolarizer 706. Since the beam has been rotated by the 90 degree rotator708 from the S-polarization to the P-polarization, the P-polarized beamis transmitted by polarizer 706 to 90 degree rotator 708 to proceedthrough the ring counter-clockwise a second time. However, during thissecond pass through the ring, 90 degree rotator 708 rotates thepolarization by 90 degrees back to the S-polarization. Therefore, whenthe beam reaches the polarizer 706 at the end of a second pass throughthe ring, it will be reflected into SBS phase conjugator 760, throughthe second intra-cavity relay telescope 770.

The beam proceeding back out of the SBS phase conjugator, still havingthe S-polarization, but reversed phase error, will be reflected bypolarizer 706 in a clockwise direction to mirror 717 where it willproceed along path 742 through telescope 720 to mirror 716. From mirror716, the beam will proceed through slab 750 a first time and bereflected back through the slab 750 a second time by mirrors 714 and715. Proceeding out of slab 750, the beam will be reflected off mirror713 and proceed back through telescope 720 and mirrors 712 and 711 to 90degree rotator 708. The 90 degree rotator 708 will rotate thepolarization by 90 degrees back to the P-polarization and transmit thebeam to polarizer 706, thus completing a third pass through the ring,but this time in the reverse direction from the first two passes.

Since the beam has a P-polarization, the beam will pass throughpolarizer 706 and proceed clockwise through the ring for a fourth passthrough the ring, or a second pass in the reverse direction. At the endof this fourth pass through the ring, the 90 degree rotator will rotatethe polarization back to the S-polarization causing the beam to reflectoff of polarizer 706 out of the ring and into isolation rotator 740. Bythis point, the net accumulated phase error is essentially zero,providing a wavefront corrected output pulse. The isolation rotator 740will rotate the polarization of the beam to the P-polarization enablingthe beam to pass through polarizer 702 as a high energy output pulse.

Thus, the beams passing through the amplifier illustrated in FIG. 7exhibit reduced diffraction, minimizing the likelihood of high peakperturbations in a beam, by utilizing two paths around the ring beforeentering the phase conjugator, and two equal and opposite paths around aring after exiting the phase conjugator. The ring, further, utilizes apassive polarization rotator instead of a Pockels cell. Additionally,all optical components within the resonator are placed near the imageplanes by the use of relay telescopes (two paths through firstintra-cavity telescope 720 and of the second intra-cavity telescope770). The amplifier also exhibits higher gain-to-loss ratio, with twoslab passes providing gain in each ring transit. The SBS phaseconjugator acts as a mirror system and offsets phase aberrations in thebeam. In embodiments of the invention, the SBS phase conjugator/mirrorsystem 760 includes components used for pulse width control, used as analignment fiducial for the optical path in the ring, and which limitself-focusing and other aberrations induced by SBS media.

The single-frequency master oscillator 708 in FIG. 7 in one preferredembodiment, comprises a relaxation pulse-seeded oscillator, whichprovides consistent single-frequency with good amplitude and temporalstability, with representative pulse profiles having a pulse height ofgreater than 1.2 megawatts and a pulse width of about 24 nanosecondsfull width half maximum. Other master oscillator embodiments can be usedas mentioned above. The relaxation pulse-seeded oscillator in oneembodiment includes a laser resonator having an output coupler and anumber of other reflectors defining an optical ring, preferably havingan odd total number of reflectors including the output coupler. AQ-switch and a gain medium are included in the resonator. A detector iscoupled with the resonator to detect oscillation energy in theresonator. A controller is coupled to a source of energy for the gainmedium, to the Q-switch, and to the detector. A component in theresonator induces loss while building up gain in the gain medium withthe source of pump energy, until a gain-to-loss ratio is achieved thatis sufficient to produce a relaxation oscillation pulse. Upon detectionof an onset of the relaxation pulse, the controller decreases the lossusing the Q-switching so that an output pulse having a single frequencyis generated. A set of etalons in the resonator restricts oscillation toa single longitudinal cavity mode during the onset of the relaxationoscillation pulse. Also, a transverse mode limiting aperture is placedin the laser resonator.

A laser peening method and system, and laser energy delivery systemsuitable for such use and other uses, which allows the work piece to befixed, while moving and directing the laser beam in a highly controlledmanner is provided. Utilizing a fixed work piece position minimizesholding fixture and work piece moving complexity, thereby reducing costand engineering. Moving the laser beam rather than the work piece willallow work pieces and structures larger than automation handlingcapacity to be cost effectively laser peened.

A system as described herein provides sufficient flexibility to be ableto treat large work pieces “in situ” at customer facilities, for exampleaircraft at an aviation repair station or large oil drilling work piecesat a pipe yard. A particular application of interest is that of laserpeen forming, requiring parts that can be many tens of meters in lengthto be treated.

In general, a methodology and associated mechanisms to allow controlledmovement of high energy laser beam pulses across the surface of variouswork pieces, the work piece held in the fixed position during processingis provided which maintains the laser beam's near field spatial profileand image and shot position orientation with reference to other lasershots. The system is capable of creating a field of compressive residualstress on a work piece surface and subsurface by laser peening using awell-defined pattern of laser shots, often arranged into rows ofadjacent shots. Each shot must be placed at the proper work piecelocation by careful control of laser beam incidence angle. Beam imagelocation and beam rotation indicates the square or rectangular beam.Movement of the work piece subject of laser peening or other processingis reduced or eliminated in systems applying the present technology.

Representative uses of the described technology include laser peeningsmall and large work pieces and structures, laser peening forming, laserbeam delivery for other surface modifications such as heat treatment,texturing, cutting and welding. The system is adaptable for beamdelivery for materials processing application of laser energy, includingbut not limited to including superplastic forming, paint or other typeof coating removal, etching, engraving and marking.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims.

What is claimed is:
 1. An apparatus for delivering a sequence of pulsesof laser energy from a source of laser energy to a target surface on awork piece, comprising: beam delivery optics having adjustablecomponents arranged to receive the laser energy from an output of thesource of laser energy and direct the laser energy along an optical pathtoward the target surface, the optical path including a variable segmentbetween first and second components of the beam delivery optics having avariable length and variable direction through air and establishing anoutput beam line, wherein the first component is adjustable to define adirection of the variable segment and a second component is adjustableto define a length of the variable segment and an angle of incidence ofthe laser energy on the second component; and a controller, coupled toadjustable components of the beam delivery optics, to adjust the firstcomponent to change the direction of the variable segment and to adjustthe second component to change the length of the variable segment and anangle of incidence of the laser energy on the second component, in orderto move the output beam line for the laser energy among target positionson the target surface; and wherein the controller moves the output beamline to cause a plurality of pulses in the sequence of pulses to impactthe target surface in a pattern of spots in a first process area on thework piece without changing the length of the variable segment.
 2. Theapparatus of claim 1, including a diagnostic sensor coupled with thebeam delivery optics and to the controller for closed-loop optimizationof pointing angles for the output beam line.
 3. The apparatus of claim1, including a diagnostic sensor which senses an image of the laserenergy on the target surface.
 4. The apparatus of claim 1, including adiagnostic sensor which senses position of the target surface relativeto the beam propagation optics.
 5. The apparatus of claim 1, wherein thebeam delivery optics comprise: input optics arranged to receive thelaser energy from an output of the source of laser energy and to directthe laser energy on a first optical path segment; the first componentincluding a transmitting mirror having an adjustable angle of incidencerelative to the first optical path segment and reflecting the laserenergy on a second optical path segment, the second optical path segmentcomprising the variable segment, the second component including a robotmounted optical assembly, the robot mounted optical assembly including areceiving mirror adapted to be positioned in the second optical pathsegment and having an adjustable angle of incidence relative to thesecond optical path segment to reflect the laser energy on a thirdoptical path segment, and an output telescope in the third optical pathsegment directs the laser energy toward the target surface on the outputbeam line, and wherein controller moves the output beam line to cause aplurality of pulses in the sequence of pulses to impact the targetsurface in a pattern of spots in a first process area on the work piecewithout changing the location of the robot mounted optical assembly. 6.The apparatus of claim 5, wherein the input optics comprise a telescopearranged to project the laser energy along the first optical path. 7.The apparatus of claim 5, wherein the input optics comprise a telescopearranged to enlarge a cross-section of the laser energy and project thelaser energy along the first optical path for propagation with theenlarged cross-section along the variable segment.
 8. The apparatus ofclaim 5, including a diagnostic sensor which senses alignment of thelaser energy with receiving mirror.
 9. The apparatus of claim 5,including a diagnostic sensor which senses the adjustable angle ofincidence of the receiving mirror.
 10. A method for delivering laserenergy from a source of laser energy on an adjustable output beam lineto a target surface on a work piece, comprising: receiving the laserenergy from an output of the source of laser energy on a firstcomponent; controlling the first component to direct the laser energy inan adjustable direction; positioning a second component in an adjustablelocation to receive the laser energy from the first component andcontrolling the second component to define an adjustable angle ofincidence of the laser energy on the second component to define avariable segment of an optical path between the first and secondcomponents the variable segment having a variable length and variabledirection; and directing a plurality of pulses of the laser energyincident on the second component at the adjustable location, so that theplurality of pulses impact more than one target position within a firstprocess area on the target surface without changing the adjustablelocation; changing the adjustable direction, the adjustable location andthe adjustable angle of incidence to move the adjustable output beamline to a second process area on the target surface.
 11. The method ofclaim 10, including utilizing a camera mounted with the second componentto locate and calibrate a position of the work piece relative to thesecond component.
 12. The method of claim 10, including sensing an imageof the laser energy on the target surface.
 13. The method of claim 10,including sensing a position of the target surface.
 14. The method ofclaim 10, wherein the first component comprises a transmitting mirrorand said controlling a first component, includes: adjusting thetransmitting mirror to reflect the laser energy on a second optical pathsegment, the second optical path segment comprising the variablesegment, and wherein the second component comprises a receiving mirroron a robot mounted optical assembly and said controlling the secondcomponent to define an adjustable angle of incidence includes adjustingthe receiving mirror to reflect the laser energy on a third optical pathsegment, wherein the third optical path segment directs the laser energytoward the target surface on the output beam line.
 15. The method ofclaim 10, including sensing alignment of the laser energy with receivingmirror.
 16. The method of claim 10, including sensing the adjustableangle of incidence of the receiving mirror.
 17. The method of claim 10,including enlarging a cross-section of the laser energy for propagationwith the enlarged cross-section along the variable segment.
 18. Themethod of claim 10, including using a diagnostic sensor for closed-loopcontrol of the moving of the adjustable output beam line.